unicode-monkeypatch: use tostring() in io.open error handling

msg can be nil, which would previously error out when trying to print it.

automation/include/unicode-monkeypatch.lua

@@ -100,7 +100,7 @@ function io.open(fname, mode)
 	local file = assert(orig_open("nul", "rb"))
 	if ffi.C._wfreopen(wfname, wmode, file) == nil then
 		local msg, errno = select(2, file:close())
-		return nil, fname .. ": " .. msg, errno
+		return nil, fname .. ": " .. tostring(msg), errno
 	end
 
 	return file